The proposed studies are intended to define and characterize "structural domains" of rhodopsin in photoreceptor disc membranes and to determine whether such domains participate in particular biochemical functions. This will provide information about the location, extent and structure of "functional domains" in rhodopsin. By integrating these two ways of studying this protein, it will be possible to more directly approach the question of what rhodopsin's specific roles are in visual excitation and control of enzymic activities in photoreceptor cells. Specific protein modification techniques (including proteolysis, chemical cleavage, sulfhydryl modification) will be applied to native and inverted disc membrane vesicles. The modified disc membranes produced in these experiments will represent varying degrees of degradation and perturbation of rhodopsin structure and will be used to 1) identify regions of rhodopsin exposed at intradiscal and extradiscal surfaces, 2) isolate transmembrane peptides, 3) determine regions of the protein which are required for the activation of cGMP-dependent phosphodiesterase or for the stimulation of rhodopsin phosphorylation in response to light. These studies will elucidate the molecular mechanisms of these activation processes. Since regulation of cGMP levels is a requirement for the normal development of the visual system, understanding the control of cGMP phosphodiesterase may shed light on various disease states of photoreceptor cells.